Generally, in an AC servomotor, when reverse revolution braking (sic) is applied to the motor, a voltage is regenerated from the motor to a driving circuit, and the voltage on a smoothing capacitor provided on an input of the driving circuit rises, and sometimes the voltage exceeds the rated voltage of the capacitor.
Therefore, a method wherein, when the voltage of the smoothing capacitor rises, the voltage is detected and is discharged to a discharge circuit and the voltage rise of the smoothing capacitor is restricted, is known.
Hereafter, referring to the drawings, a conventional motor control apparatus is elucidated.
FIG. 1 shows a configuration of the conventional motor control apparatus. Referring to FIG. 1, numeral 1 designates an alternating current power source, numeral 2 designates a rectifier diode, numeral 3 designates a smoothing capacitor for smoothing an output of the rectifier diode 2, numeral 4 designates a motor driving circuit wherein a terminal voltage of the smoothing capacitor 3 is inputted, numeral 5 designates a servomotor which is driven by an output of the motor driving circuit 4, numeral 6 designates a regenerative voltage detection circuit which is connected to the input of the driving circuit 4, numeral 7 designates a discharge circuit of the smoothing capacitor 3, numeral 8 designates a protection circuit of the discharge circuit 7, numeral 9 designates an overvoltage detection circuit for stopping operation of the motor driving circuit 4 when an input of the motor driving circuit 4 exceeds a predetermined level, numeral 10 designates a voltage drop detection circuit for stopping operation of the motor driving circuit 4 when the input of the motor driving circuit 4 is lowered below a predetermined level during the operative state of the motor driving circuit 4.
Operation of the motor control circuit is hereafter described in more detail.
FIG. 2 shows voltage detecting levels Va, Vb, Vc of the regenerative voltage detection circuit 6, the overvoltage detection circuit 9 and the voltage drop detection circuit 10, respectively. In the same figure, V.sub.DC designates a terminal voltage of the smoothing capacitor 3.
When the motor turns into a braking state, V.sub.DC rises gradually, and when V.sub.DC &gt;Va is attained, the regenerative voltage detection circuit 6 is activated, and the discharge circuit 7 is connected to the smoothing capacitor 3, and thereby the electric charge stored in the smoothing capacitor 3 is discharged through the discharge circuit 7, and V.sub.DC is reduced and returns to a steadystate. When V.sub.DC &gt;Vb is attained by an instantaneous voltage rise, the over-voltage detection circuit 9 is activated, and the driving circuit 4 is caused to suspend operation and V.sub.DC is reduced. Even if the voltage of the smoothing capacitor 3 has become V.sub.DC &lt;Vc while starting of the motor 5, the voltage drop detection circuit 10 stops the driving circuit 4 and serves to prevent drop of the voltage of the smoothing capacitor 3 under a set value.
Hereupon in the conventional apparatus, when the discharge circuit 7 is operated during a long time period, there is a danger of burning out the discharge circuit, and hence the protection circuit 8 for protecting the discharge circuit 7 works, and input to the discharge circuit 7 is intercepted; but there has been a disadvantage that the driving circuit 4 is operational in this state, and a voltage is regenerated from the motor to the driving circuit 4, and thereafter the voltage rise of the smoothing capacitor 3 can not be restricted if the voltage of the smoothing capacitor 3 did not rise until it becomes V.sub.DC &gt;Vb.